Radioactive decay (AQA GCSE Physics): Revision Notes
Radioactive decay
What is radioactive decay?
Some atoms have nuclei that are unstable. These unstable nuclei try to become more stable by giving out radiation. This process happens randomly and is called radioactive decay.
When radioactive decay happens, the unstable nuclei release energy and particles to become more stable. You cannot predict exactly when a particular nucleus will decay, but you can measure how often decay happens in a large group of atoms.
The random nature of radioactive decay means we can only make statistical predictions about large groups of atoms, never about individual nuclei.
Activity and measuring radioactivity
Activity tells us how fast radioactive decay is happening. It measures how many unstable nuclei break down each second.
Activity is measured in becquerels (Bq):
- 1 Bq = 1 nucleus decaying per second
- 1000 Bq = 1000 nuclei decaying per second
Different radioactive sources have different activities:
- Very unstable sources might have high activity (like 1,000,000 Bq)
- More stable sources have lower activity (like 20 Bq)
The activity depends on how unstable the nuclei are and how many radioactive nuclei are present.
Activity is directly proportional to the number of unstable nuclei present - more unstable nuclei means higher activity, and vice versa.
How activity changes over time
The activity of a radioactive source gets weaker over time. This happens because:
- Unstable nuclei gradually decay into stable nuclei
- There are fewer unstable nuclei left to decay
- Eventually, most nuclei become stable
When you plot activity against time on a graph, you get a curved line that starts high and drops down. The line gets flatter as time goes on because there are fewer unstable nuclei left.
Scientists often describe this decrease using something called half-life, which tells us how long it takes for the activity to drop to half its original value.
Changes to the nucleus
When radioactive decay happens, the nucleus can change in different ways depending on what type of radiation is given out:
Nuclear Changes During Different Types of Decay
Alpha (α) radiation:
- The nucleus loses mass (goes down by 4)
- The nucleus loses positive charge (goes down by 2)
Beta (β) radiation:
- The nucleus mass stays the same (no change)
- The nucleus gains positive charge (goes up by 1)
Gamma (γ) radiation:
- No change to the mass of the nucleus
- No change to the charge of the nucleus
These changes help unstable nuclei become more stable.
Detecting radiation with a Geiger-Müller tube
Scientists use a Geiger-Müller (GM) tube connected to a counter to detect nuclear radiation. This equipment can detect alpha particles, beta particles, and gamma rays.
The count rate is the number of radiation particles detected by the GM tube in a certain time period. This tells us how much radiation is being given out.
The GM tube works by detecting when radiation passes through it, and the counter keeps track of how many particles are detected. This gives us a measurable way to study radioactive decay.
Key concepts to remember
Key Points to Remember:
- Radioactive decay happens when unstable nuclei give out radiation to become more stable
- Activity measures how fast decay happens and is measured in becquerels (Bq)
- Activity gets weaker over time as unstable nuclei turn into stable ones
- Different types of radiation (alpha, beta, gamma) change the nucleus in different ways
- Geiger-Müller tubes are used to detect and measure nuclear radiation